Two U.S. appeals courts issued conflicting rulings on whether consumers can get subsidies for health coverage bought on the Affordable Care Act's federal exchange, escalating a legal battle that could complicate fall insurance enrollment and jeopardize tax credits for millions of Americans.

In a blow to President Barack Obama's signature legislative achievement, a panel of the U.S. Court of Appeals for the District of Columbia Circuit, on a 2-1 vote, invalidated an Internal Revenue Service regulation that implemented a...